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Subject[tip:x86/mm] x86, mm: Use a bitfield to mask nuisance get_user() warnings
Commit-ID:  b390784dc1649f6e6c5e66e5f53c21e715ccf39b
Gitweb: http://git.kernel.org/tip/b390784dc1649f6e6c5e66e5f53c21e715ccf39b
Author: H. Peter Anvin <hpa@zytor.com>
AuthorDate: Mon, 11 Feb 2013 16:27:28 -0800
Committer: H. Peter Anvin <hpa@zytor.com>
CommitDate: Mon, 11 Feb 2013 17:26:51 -0800

x86, mm: Use a bitfield to mask nuisance get_user() warnings

Even though it is never executed, gcc wants to warn for casting from
a large integer to a pointer. Furthermore, using a variable with
__typeof__() doesn't work because __typeof__ retains storage
specifiers (const, restrict, volatile).

However, we can declare a bitfield using sizeof(), which is legal
because sizeof() is a constant expression. This quiets the warning,
although the code generated isn't 100% identical from the baseline
before 96477b4 x86-32: Add support for 64bit get_user():

[x86-mb is baseline, x86-mm is this commit]

text data bss filename
113716147 15858380 35037184 tip.x86-mb/o.i386-allconfig/vmlinux
113716145 15858380 35037184 tip.x86-mm/o.i386-allconfig/vmlinux
12989837 3597944 12255232 tip.x86-mb/o.i386-modconfig/vmlinux
12989831 3597944 12255232 tip.x86-mm/o.i386-modconfig/vmlinux
1462784 237608 1401988 tip.x86-mb/o.i386-noconfig/vmlinux
1462837 237608 1401964 tip.x86-mm/o.i386-noconfig/vmlinux
7938994 553688 7639040 tip.x86-mb/o.i386-pae/vmlinux
7943136 557784 7639040 tip.x86-mm/o.i386-pae/vmlinux
7186126 510572 6574080 tip.x86-mb/o.i386/vmlinux
7186124 510572 6574080 tip.x86-mm/o.i386/vmlinux
103747269 33578856 65888256 tip.x86-mb/o.x86_64-allconfig/vmlinux
103746949 33578856 65888256 tip.x86-mm/o.x86_64-allconfig/vmlinux
12116695 11035832 20160512 tip.x86-mb/o.x86_64-modconfig/vmlinux
12116567 11035832 20160512 tip.x86-mm/o.x86_64-modconfig/vmlinux
1700790 380524 511808 tip.x86-mb/o.x86_64-noconfig/vmlinux
1700790 380524 511808 tip.x86-mm/o.x86_64-noconfig/vmlinux
12413612 1133376 1101824 tip.x86-mb/o.x86_64/vmlinux
12413484 1133376 1101824 tip.x86-mm/o.x86_64/vmlinux

Cc: Jamie Lokier <jamie@shareable.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20130209110031.GA17833@n2100.arm.linux.org.uk
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
---
arch/x86/include/asm/uaccess.h | 20 +++++++++-----------
1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 1e96326..a8d1265 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -168,31 +168,29 @@ do { \
#define get_user(x, ptr) \
({ \
int __ret_gu; \
- unsigned long __val_gu; \
- unsigned long long __val_gu8; \
+ struct { \
+ unsigned long long __val_n : 8*sizeof(*(ptr)); \
+ } __val_gu; \
__chk_user_ptr(ptr); \
might_fault(); \
switch (sizeof(*(ptr))) { \
case 1: \
- __get_user_x(1, __ret_gu, __val_gu, ptr); \
+ __get_user_x(1, __ret_gu, __val_gu.__val_n, ptr); \
break; \
case 2: \
- __get_user_x(2, __ret_gu, __val_gu, ptr); \
+ __get_user_x(2, __ret_gu, __val_gu.__val_n, ptr); \
break; \
case 4: \
- __get_user_x(4, __ret_gu, __val_gu, ptr); \
+ __get_user_x(4, __ret_gu, __val_gu.__val_n, ptr); \
break; \
case 8: \
- __get_user_8(__ret_gu, __val_gu8, ptr); \
+ __get_user_8(__ret_gu, __val_gu.__val_n, ptr); \
break; \
default: \
- __get_user_x(X, __ret_gu, __val_gu, ptr); \
+ __get_user_x(X, __ret_gu, __val_gu.__val_n, ptr); \
break; \
} \
- if (sizeof(*(ptr)) == 8) \
- (x) = (__typeof__(*(ptr)))__val_gu8; \
- else \
- (x) = (__typeof__(*(ptr)))__val_gu; \
+ (x) = (__typeof__(*(ptr)))__val_gu.__val_n; \
__ret_gu; \
})

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